1. Technical Field
The present invention is directed to a process for fabricating vertical transistors.
2. Art Background
In integrated circuits, there is a trend toward a higher device density to increase the number of devices per unit area. Device density is increased by making individual devices smaller and placing the devices closer together. Device dimensions (termed feature size or design rules) are decreasing from 0.25 .mu.m to 0.18 .mu.m and beyond. It is also desired to decrease the distance between devices in a commensurate fashion.
Currently, most MOS (metal oxide semiconductor) transistors have a planar configuration. In a planar MOS device, the direction of the current flow is parallel to the plane of the substrate surface. Although there is a need to decrease the size of these devices to achieve increased device density, fabricating these small devices becomes increasingly difficult. In particular, lithography becomes extremely difficult as device dimensions decrease to less than the wavelength of the radiation used to delineate an image of a pattern in a radiation-sensitive material.
A vertical device configuration, described in Takato, H., et al., "Impact of Surrounding Gate Transistor (SGT) for Ultra-High-Density LSI's" IEEE Transactions on Electron Devices, Vol. 38 (3), pp. 573-577 (1991) has been proposed as an alternative to the more space-consuming planar device configuration. A schematic of the device is illustrated in FIG. 1. The device 10 has a source 15, drain 20, and channel 25. The length of the channel 25 is perpendicular to the surface of the substrate 30 on which the device 10 is formed. The device is called a vertical transistor because the length of the channel is perpendicular to the substrate surface. A gate 35 surrounds the channel 25.
Although vertical MOSFETs (metal oxide semiconductor field effect transistors) can be packed more densely than planar MOSFETs, the processing issues for the vertical transistors are not trivial. A process that makes it easier and more efficient to fabricate vertical MOSFETs is therefore desired.